Various technical means are known today for tracking and authenticating a product. This product may be an art object, a commercial consumer product, an identity document, a means of payment, etc.
Such a product may therefore consist of one or more materials, which include for example, paper, textile, leather, plastic, etc. Among the means employed today to trace and authenticate such products, mention can be made of the barcode, holograms which must be decoded optically, electronic labels (sometimes denoted by the acronym RFID for “Radio Frequency Identification Data”) which are decoded by an electromagnetic method. Also known are cryptography techniques which consist in attributing an encrypted numerical code to the product to be authenticated, or the insertion of specific elements such as tracers (luminescent, magnetic, chemical, etc.) in the very structure of the object or the product to be authenticated.
The present invention relates to the latter field and, more particularly, the method for tracing or authenticating a product by means of luminescent tracers.
In a manner known per se, such luminescent tracers form an optical code which must be read in order to authenticate the object or product. In general, a mixture is applied comprising one or more luminophores, that is small grains, molecules or particles of light emitting material, directly to the surface of the product to be authenticated.
During the authentication of the product, an external luminescent source is used to excite the luminophores thus deposited, and the light rays that they re-emit by deexcitation are collected. The analysis of the luminous spectra re-emitted by these luminophores serves accurately to determine the optical code which has been attributed to the object to be authenticated associated with these luminophores.
Document U.S. Pat. No. 4,146,792 provides an example of such an optical coding designed to authenticate fiduciary instruments. In a manner known per se, the electrons of the atoms of the luminophore are excited by the photons of a light source, and then, during their deexcitation, emit photons whereof the wavelength depends on that of the incident rays. In this case, the fluorescence spectrum emitted by the luminophore is partially absorbed by a dye, so that a purely photon mechanism is involved. It is therefore difficult to authenticate the nature of such luminophores. Thus, the authentication of the fiduciary instrument is much more reliable.
However, luminophores designed for such authentication applications must have a high luminescence yield (defined as the ratio of the number of photons emitted to the number of photons absorbed by the luminescent material) and must be sufficiently robust to be durably able to emit by luminescence. In fact, luminophores having these yield and robustness properties exist in limited numbers, so that the possible combinations for forming optical codes are also limited in number. Insofar as the fluorescence spectra, particularly the peaks, are already known and listed, it is then relatively easy to identify the luminescent materials making up the luminophores used, and thereby to counterfeit the optical code.
Moreover, in the rare cases in which the luminescent material employed is relatively unknown, it is relatively easy to analyse the physicochemical spectrum of the material comprising the luminophores. This makes it possible to fake the optical coding device and thereby falsify authentic objects.
Document US 2005/142605 describes the use of nanoparticles having a surface plasmon effect, to increase the absorption or fluorescence properties of chemical substances intended for marking a medium. Thus, it is possible to apply a lower excitation to light sensitive materials.
It is therefore an object of the present invention to provide an optical coding device that is unfalsifiable and unique. This optical coding device has the basic purpose of avoiding the drawbacks of the prior art, and particularly of preventing the retrospective analysis of the luminous spectrum re-emitted by the luminescent aggregates associated with the object to be authenticated.